I got the Vox board + OT + tube, guess what...I prefer the Valve Jr stock amp!

[cGil]

The Fender board would be closer, but still, just tweak what you've got! It's really not that far from what you want it to be when you consider that a few simple component swaps is all it'd take to get it there.

 

Gil...

[BadAssBluesDude]

Awwwww, but I wanted to buy more stuff. Seriously I have to get busy, the reason I'm doing all this amp stuff is that after 12 years of being a solo act using my digital modeler, I'm joining a band so I need a live rig. There's some neurosis inside of me that won't let me just be happy with over the counter stuff, I have to say "I made that". I'm building my own pedals too.

 

I'm putting in a tone knob and the NFB/C3 switch today.

 

Another question regarding what Gil said about using a 22k or a 27K for the NFB. What would be the difference I hear between using a 22k or the 47K I see on the Fender Mods?

 

I have 1/2 watt resistors at my house, can I use one of those in the NFB until I can replace it with a 1 watt?

 

Also concerning the NFB, my Vox modded board has an unshielded wire running from the 7 post of the preamp tube to the to the 3 post on the volume pot. In diagrams I've seen they call for a shielded coax cable with the ground going to the back of the pot. Is this a big enough deal to mess with?

 

Also, Cole talked about doing a switch on C4 and I've noticed that the Fender boards don't even have a C4. What would be the effect of having a C4 lift switch?

 

As always thanks so much everyone. My wife is really pissed at all of you for enabling me in my new addiction.

 

TC

[cGil]

On the NFB resistor, 22k is real thin, 27k is a little thin, and 47k is just a touch of thinness. Try a 5k or 10k resistor connected to the jack, and a 50k linear pot as a variable resistor between the fixed resistor and the NFB switch. (Or a 22k resistor and a 25k pot, or just whatever you have on hand) Then you can dial up any amount of NFB you want.

 

As for the 1/2 watt resistor... Try it. But if you're worried, then measure the voltage across it and calculate the current and the power it's dissipating. If it's dissipating close to 1/2 a watt or more, it won't live long and you'll need to get a 1w resistor (or higher) in there soon. I admit I haven't bothered to do this, but then, I use 1watt resistors and haven't seen the need to worry about it.

 

Shielded wire is definitely a good idea to use between the volume pot's wiper (#2 lug) to the 12ax7's pin 7 (input grid) because the volume pot strangles the signal strength, and that lowered signal then becomes susceptible to picking up noise in the tight quarters of that chassis. I built an 18watt without a single strand of shielded wire, and it's utterly quiet; but proper layout and lead dress are even more of an absolute critical necessity for the amp to work like that.

 

Gil...

[Cole]

Wow i've just been modding my amp on instructions given mostly by cGil and now i can offer some advice! I don't know why the fender doesn't have a C4 but off of cGils advice i put in a marshall/fender switch for C4 with 1uf on one side and 5uf on the other. It's very subtle but really helps. The 1uf marshall side has a very tight bass responce and sounds best dirty. the 5uf fender side has a more rounded bass and sounds better clean. Hope that helps!

 

ask gil about it. He knows WAY more than i do about it.

[BadAssBluesDude]

Yeah, I'm pretty much to the point where Gil has helped me out so much that I'm going to have to name my next child after him.

 

If I temporarily use a 1/2 watt resistor and it does burn out, could this damage anything other than the resistor?

 

Silly question, which end is the + end of the C3?

 

I'm going with the 10K resistor and 50k pot setup. I like the idea of being able to dial in a sound.

 

TC

[cGil]

Yeah' date=' I'm pretty much to the point where Gil has helped me out so much that I'm going to have to name my next child after him.

[/quote']

Ain't even born yet and he's scheduled for abuse. That poor child!

 

If I temporarily use a 1/2 watt resistor and it does burn out' date=' could this damage anything other than the resistor?

[/quote']

 

Probably not. Resistors tend to burn "open" as opposed to turning into a dead short, so if you flip the NFB in and it doesn't sound like anything changed, the resistor may have burned out, or open as the case may be.

 

Silly question' date=' which end is the + end of the C3?

[/quote']

 

The other end! The negative lead on radial can caps is usually marked by a black stripe on that side of the can. If it's an axial cap (leads at opposite ends of the cylinder) it may have arrows on the black stripe pointing toward the "-" lead (if it even has a black stripe; some don't!). It will have an indentation circling the end of cap on the "+" end. All axial caps that I've seen use that indentation marker for the positive lead. Radial can caps don't have the dent, so you must rely on the black stripe for those.

 

I'm going with the 10K resistor and 50k pot setup. I like the idea of being able to dial in a sound.

 

TC

 

Rock on pecan.

 

Gil...

[BadAssBluesDude]

This is an edited post because when I first wired everything up I had huge problems. I took one part at a time out, until I was back to my starting point and then put one part at a time back in and now everything is working just fine.

 

Riddle me this.......

 

The C4 "in" position I way more gain than I will need with the band I'm going to be playing in. Would it be feasible to wire the NFB in/out to a stereo jack and then using a TRS cable run to a footswitch so that I could control the NFB on/off with my feet?

 

I'm still wanting to tame my gain so I might have to try a 12at7, or a 12ay7.

 

Now to hook up my tone knob and effects loop. It's a Loop-D I got on Ebay. Gil didn't you make these? Now that I see what it is I realized I could have easily made one myself.

 

TC

[BadAssBluesDude]

Well I knew it couldn't last. I tried to install a C4 lift switch and now I have no gain. It's down to about 5% of what it should be.

 

I had a hard time getting the C4 + lead out of the 3 pin of the tube socket. Is it possible that having the soldering iron on the tube socket lug too long damaged it?

 

I took the C4 switch back out and still 5% gain.

 

Back to work....

 

TC

[BadAssBluesDude]

Well, I've done something bad, I've got a dead amp. I removed C3 & C4 and removed the NFB switch and just direct wired a 22k resistor and I've not nothing.

 

I powered the amp up and took the cover off the preamp tube and it doesn't look like it's glowing at all. Like I said before, I'm really thinking I left my solder iron on the tube lug too long and messed something up.

 

Oh well, it's nothing that can't be fixed.

 

I'll keep you posted.

 

TC

[Eracer_Team]

You mention the preamp tube.. does the power tube light or not?

the fuse holders aren't the best.. try taking the fuses out of the orange wires and scraping the contacts in the fuse holders and put them back together before doing any thing else.

[BadAssBluesDude]

Thanks, I'll try that. The EL84 does light up. Being new to tube amps I'm not sure how bright it should light up, but it is glowing. I'll try the fuse idea.

 

Thanks,

TC

[BadAssBluesDude]

I was wrong to use the term dead amp, like I said there's a small signal getting through.

 

The preamp tube is glowing and getting warm. If I take the cover off I get grounding/interference noise.

 

I really think I fried the tube socket lug.

 

This is still fun though.

 

TC

[cGil]

Also, those quick connects all over the board for the transformer connections are very fragile. Merely tugging on a connector can break the solder connection underneath the lug. I pry 'em apart with a poker of some sort so they just slide off real easy and crimp 'em a little when I put 'em back on so they make a good pressure contact. Might want to try reflowing the solder on those quick connect lug solder pads.

 

Any preamp cathode bypass cap in the circuit, whether C3 or C4, will increase the gain for its respective stage. The higher the value up to 22F, the more apparent gain you'll have because the higher values will allow more bottom end response in that stage's tone.

 

Gil...

[BadAssBluesDude]

Gil, I think you're forgetting that I'm not modding a stock board, it's an eyelet board from Turretboards.com. All the plugs have been hardwired with Switchcraft plugs. The only "quick connects" I see are the ones the to AC plug and On/Off switch.

 

I can post a picture if this would help.

 

Is there any way to test what is or isn't happening with the suspect #3 preamp tube lug?

 

TC

[cGil]

Yep. Forgot. That's the problem with long threads and a memory that grows shorter with each passing day. Or, maybe my mother's senility is contagious? Okay, so I guess we've eliminated those problematic quick connects as a possibility.

 

 

Anyway, pin 3 is the first gain stage's cathode, so there should be ~1.3 to 1.5vDC on it with a 1.5k cathode resistor at idle.

 

If you probe your way with a meter from the power tube pins toward the first gain stage's tube pins, the pops from the speaker should get louder as you go. If the pops stop, the problem is nearby, unless it's because the volume knob is turned all the way down. So crank it up for this test.

 

Gil...

[BadAssBluesDude]

Gil, I hope my next child is a boy, I'd feel bad naming a girl Gil.

 

Here's what I got with the amp turn on full, speaker plugged in, and no guitar cable plugged in.

 

preamp tube pin 1---180 volts dc---small click, not loud enough to be a pop

pin 2--------------------no voltage-----no pop or noise

pin 3--------------------1 volt dc--------no pop or noise

pin 4&5--------------------no voltage-----no pop or noise

pin 6--------------------190 volts dc---loud pop

pin 7--------------------no volts--------ground buzz

pin 8--------------------1 volt dc-------medium pops

pin 9--------------------no volts--------no pops

 

Does this tell you anything? I've put everything back to the way it originally was except I left out C3. Now the amps volume/gain when cranked fully is maybe at 10%.

 

Here's an odd ball question. I've obviously screwed up something in the preamp section and might have to wait for new parts. If I went ahead and installed my effects loop, would I still be able to use the output section with a digital modeler in front?

TC

[Eracer_Team]

yup those voltage's tell us something.. no volt's on 4&5 not good that's your heater line to the 12ax7 and should be 5.75~6.90vdc (watch out some turret boards are a/c voltages).

Pin 1 should be around 160~210vdc depending on R8,

Pin2 is input so nothing is good

Pint3 can be from .89 to 2.10 (most of the time between .9~1.5 depending on R8 value.)

Pin 4&5 should be the 5.75~6.90 vdc BUT.. if you're running the turret board it might be A/C heaters so switch to a/c and check again

Pin 6 160~210vdc depending on R9 and you should hear a big POP on touching this pin.

Pin 7 input to second stage so again no voltage.

Pin8 should be just like pin 3 can be from .89 to 2.10 (most of the time between .9~1.5 depending on R9 value

 

 

Loud pop on pin6 sort of tells us that the EL84 should be okay.

But it doesn't hurt to tell us what are the voltages like on the EL84 too.

Pin3, pin 7, pin9

 

Watch the R6/R7 relationship.. I think the Turret board pdf had those labeled backwards.

Also watch out for your effects loop that you haven't disconnected the signal path with a ground out type of jack.

[BadAssBluesDude]

I haven't installed my effect loop yet. All I've messed with to get me in this pickle has been C3 & C4 & NFB. Everything was fine after installing the NFB/C3 switch, everything went belly up after trying to install a C4 lift switch.

 

 

 

Switched to A/C, pin 4&5 are showing 3-3.2 A/C volts

 

 

Thanks

TC

[BadAssBluesDude]

Here's a pic of my board in it's current state if a visual helps.

 

 

TC

[cGil]

Safety first! If you have an insulated clip for one of your meter probes, you can clip one lead on and keep one hand in your pocket while you use your free hand for the other meter probe. If you don't have any of these meter probe clips, get some. Rat Shack's got 'em.

 

One thing you'll need to remember is that your board uses AC voltage on the heater filaments; so, you can't get a good reading measuring from one heater lead to ground. The only way to accurately measure it is to set your meter for AC volts and use both probes, one on each Orange wire. The tube-side of those center tap simulating 100R resistors off the el84's pin 4 and 5 are an easy place to attach the probes.

 

As for the fx loop. That'll need to be on the front panel, and next to the volume pot if at all possible to keep the leads short and quiet. Gotta plan ahead when you're drilling your panel into swiss cheese. With a single TRS (stereo) jack for the loop, you can plug halfway in and use the fx return to feed your modeler directly into the amp, or you can plug it into the regular guitar input if it sounds better to you that way.

 

As to why the pops are quiet on the first stage of the preamp, check your signal routing connections between pin 1 and pin 7. That includes C1 to R6 to the volume pot and back. From here, I don't see a wire from the R6/R7 junction to the volume pot's input. Instead, it looks like a blue wire is running from R3/C1 jnction to the pot. That's wrong. That junction should feed pin 1. The C1 coupling cap filters out the DC from the plate and passes your signal to R6, and from there the signal is fed to the volume pot. Normally for this amp, anyway.

 

 

Gil...

[layboomo]

Well a couple things right of the bat...those orange heater wires need to have a tight twist on them to prevent noise and I would also re-do the purple wires with a twisted pair as well. Next I don't see your front star ground chassis connection? Look at the layout pics from Ken to get an idea of how it should look. It also looks like the other pot and switch go nowhere?

[BadAssBluesDude]

layboomo....Thanks, I'll wrap up those wires. I've got a pic of what I think is one of your boards so I see what you mean. The pot and switch to nowhere were the C3/NFB switch with an NFB adjustment pot.

 

Gil, if I have twins I'm going to have to name them both after you. (this is my brother Darrell and my other brother Darrell.)

 

The volume pot lead had broken completely off and I didn't notice because I was so busy looking at just the stuff I had touched.

 

Now to redo the C3/NFB switch, C4 lift, put in my tone knob and my effects loop.

 

Thanks so much everyone. You guys ROCK!!!

 

TC

[BadAssBluesDude]

Finally finished and everything I working perfectly.

 

Thanks so much to everyone.

 

I still want to do some tone tweaking. But I'm really happy with what I've got. I started with the turretboard.com Vox board with Heyboer OT, then I put in a 5751 tube, a C3/NFB switch with a NFB control pot, a C4 switch and an effects loop.

 

I love all the options I have now, so many new sounds for just a few bucks in parts. Had I not made a few mistakes the whole think could have been done in 2-3 hours.

 

I can't thank you guys enough. I have a new question, but this thread is finished.

 

TC

[cGil]

Hey man, you did all the work, and YOU fixed it, not us! So give yourself a big pat on the back for hanging in there long enough to figure it out. You deserve the congratulations cuz your amp works, you've earned good tone the hard way, and you picked up some useful skills along the way. It's a big win, win, win situation! Way to go!

 

Gil...